The present invention relates to an electrophotographic photoconductor comprising a substrate, an electroconductive layer formed on the substrate, and a photoconductive layer formed on the electroconductive layer, with an improved curl prevention layer which is formed on the back side of the substrate opposite to the electroconductive layer with respect to the substrate, and more particularly to an electrophotographic photoconductor of the above-mentioned type, in which the curl prevention layer comprises a polysulfone resin or a polyarylate resin.
In the photoconductive layer of conventional electrophotographic photoconductors, inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide are widely employed. These inorganic photoconductive materials have various shortcomings in spite of their advantages over other photoconductive materials.
For example, a selenium photoconductive layer has the shortcomings that its productivity is low and the manufacturing conditions are difficult because of the necessity for vacuum deposition of selenium when forming a selenium photoconductive layer, so that its production cost is high. In addition, selenium is remarkably susceptible to heat and mechanical shocks and has the problem that it is easily crystallized on some environmental conditions.
A cadmium sulfide photoconductive layer is easily affected by moisture and causes an environmental pollution problem unless the photoconductive layer is coated with an insulating layer.
A zinc oxide photoconductive layer has the problem that its photoconductive properties deteriorate by corona charging and exposure to light while in use, because it is sensitized with a relatively weak dye such as Rose Bengale. Furthermore, in the case of the zinc oxide photoconductive layer, since zinc oxide particles are dispersed in a resin, it is difficult to obtain a photoconductive layer with a surface having satisfactory smoothness, hardness and resistance to wear.
On the other hand, photoconductive layers containing organic photoconductive materials are more flexible, and easier to produce as compared with the above-mentioned inorganic photocondutive layers, so that a variety of organic photoconductors have been proposed as having the advantage that electrophotographic properties can be obtained at a lower cost when organic photoconductors are employed, as compared with the inorganic photoconductive materials.
Specific examples of photoconductors using organic photoconductive materials are: (1) a photoconductor in which an electron transfer complex is formed by the combination of an electron donor and an electron acceptor as disclosed, for instance, in U.S. Pat. No. 3,484,237; (2) a photoconductor in which an inorganic photoconductive material is sensitized by dyes as disclosed, for instance, in Japanese Patent Publication 48-25658; (3) photoconductors in which pigments are dispersed in a positive hole or electron active matrix as disclosed, for instance, in Japanese Laid-Open Patent Applications 47-30328 and 47-18545; (4) photoconductors comprising a charge generation layer and a charge transport layer, as disclosed, for instance, in Japanese Laid-Open Patent Application 49-105537; (5) a photoconductor comprising as the main component an eutectic crystal complex consisting of a dye and a resin, as disclosed, for instance, in Japanese Laid-Open Patent Application 47-10785; and (6) a photoconductor comprising a charge transporting complex to which an organic pigment or an inorganic charge generating material is added, as disclosed, for instance, in Japanese Laid-Open Patent Application 49-91648.
Among the above photoconductors, the function-separating type photoconductors (4) are used in practice, because there are many choices in the materials in accordance with the desired photosensitivity and functions.
The charge generation layer of the function-separating type photoconductor is prepared by dispersing a charge generating material such as azo pigments, phthalocyanine type pigments, indigo type pigments and perylene type pigments in a resin binder such as polyester, polycarbonate, polyvinyl butyral and an acrylic resin, and then coating the dispersion to a substrate.
The charge transport layer is usually prepared by (i) dissolving a charge transporting material, such as triphenylamine compounds, hydrazone compounds, .alpha.-phenylstilbene compounds and pyrazoline compounds, in an appropriate organic solvent, together with a binder resin such as polyester, polysulfone, polycarbonate, polymethacrylate esters, and polystyrene, to form a solution, and (ii) coating the solution, for instance, on a charge generation layer, and (iii) drying the same.
In such a function-separating photoconductor, a charge transport layer may be placed over a charge generation layer or thereunder. In addition, an undercoat layer (or an intermediate layer) and an overcoat layer may be provided in the photoconductor when necessary.
Hereinafter an electrophotographic photoconductor comprising an organic photoconductive material as the main component is referred to as the OPC (Organic Photoconductor).
When a flexible film, such as a polyethyleneterephthalate film, is used as a substrate of the OPC, an endless-belt-shaped photoconductor can be made. When such a flexible endless-belt-shaped photoconductor is incorporated in a copying machine, it has the advantage that the degree of freedom for the mechanical design of the layout of the copying machine is expanded, as compared with a rigid photoconductor.
Further, when the above endless-belt-shaped photoconductor is structured in such a manner that the photoconductor includes a flat portion, it has another advantage that it enables high-speed copying by use of flash exposure.
Furthermore, when both the substrate and an electroconductive layer formed thereon which serves as an electrode are made transparent, the photoconductor has the further advantage that the quenching of electric charges on the photoconductor can be effectively performed by exposing the back side of the substrate to light.
Besides the above-mentioned advantages, the OPC has the advantages that it is light-weight, suitable for mass production, and can be discarded without any problem after use.
However, an OPC belt has the shortcoming that it is easily curled. When the OPC belt is curled, the predetermined distances between the OPC belt and units surrounding the OPC, such as a charging unit and a development unit in the copying machine cannot be exactly maintained. When this takes place, there occur the problems that the charged potential of the photoconductor becomes uneven in the course of charging, exact focusing cannot be performed on the photoconductor at the step of exposure, with the result that toner particles are adversely deposited on the background of the images at the step of development.
Furthermore, if the edge portion of the OPC belt is curled when the OPC belt unit is mounted on the copying machine, the OPC belt may come into contact with the above-mentioned units such as a charging unit and a development unit so that the belt might be damaged by such contact. This may considerably shorten the life of the OPC belt.
As an attempt to solve the curling problem of the OPC belt, a method of providing a curl prevention layer is proposed. Such a curl prevention layer is disclosed, for instance, in Japanese Laid-Open Patent Application 62-100764, which comprises a film-forming binder, crystalline particles dispersed in the film-forming binder, a reaction product of a two-functional chemical binder agent and the film-forming binder, and another reaction product of the two-functional chemical binder agent and the crystalline particles. This curl prevention layer, however, has the shortcomings that the curl prevention layer peels off the substrate when used in a copying machine for a long period of time due to its weak adhesive force to the substrate, and the mounting of the photoconductor unit on the copying machine is not easy, so that its handling is difficult.